Decodes text formatted using a subset of RFC2047 (the one used for
mime-encoding mail headers).

Only supports an encoding type of ‘b’ (base 64), and only supports
the character sets ISO-2022-JP and SHIFT_JIS (so the only two encoded word
sequences recognized are =?ISO-2022-JP?B?…= and =?SHIFT_JIS?B?…=).
Recognition of these sequences is case insensitive.